Don't overlook our SF3 Trend Reports, which have 1990 and 2000 data for a wide range of geographic areas.
Basic Tables: 1990 Demographic Profile Reports are available for a wide range of geographic areas and levels. An alternative way to access these 1990 profiles is to invoke the Geographic Codes Lookup application and select the geographic area using the drop-down menus. Once you navigate to the geographic entity you want, click on the FIPS code / link to get to the data applications (aka "applinks") page for the area, and then choose the link to the 1990 Demographic Profile in the Census 1990 section.
For data from earlier censuses see this page at the Census Bureau. These collections (one per census back to 1790) of primarily printed reports have been greatly enhanced (circa 2012). This is a remarkable trove of historical census data.